Should you work out for 30 minutes or an hour?

"If you're working very hard, you can do it in an hour," he said. "So the length of time is really irrelevant without looking at the intensity of the work."

The government's fitness recommendations aren't meant to keep you in the gym for hours. It says adults ages 18 to 64 should exercise at a moderate intensity for at least 2 1/2 hours each week or at a vigorous intensity for 1 hour and 15 minutes a week.

When it comes to knocking out those workouts in 30 minutes vs. 60 minutes, Smith said to think of it like sprinting vs. jogging: What you spend on intensity, you make up for in time. It's why high-intensity interval training, or HIIT workouts, are so popular right now. They can be as short as 20 minutes, and you not only burn more calories than a lower-intensity training session, but you keep burning calories in the hours after you're done.

But you need to work up to that.

If you're a rookie, a 60-minute workout could be easier because it might be tough to go at the intensity you need for a shorter time.

And as you get stronger, Smith said you can easily cut out some exercises that don't maximize your time. For example, if you want to lose weight, chin-up exercises are better than bicep curls because they work more muscle groups.